Calibration data in a sensor system

ABSTRACT

A method performed in a processing unit for determining calibration data to be used when processing data from a sensor unit connected to the processing unit, the method including receiving, from the sensor unit, an identifier which identifies and is calculated based on calibration data stored in the sensor unit, checking if there is an identifier stored in the processing unit identical to the received identifier, where the identifier stored in the processing unit identifies and is calculated based on calibration data stored in the processing unit, if an identical identifier is stored in the processing unit, using the calibration data stored in the processing unit identified by the identical identifier when processing data from the sensor unit, if no identical identifier is stored in the processing unit, requesting the sensor unit to transmit calibration data, and using the requested calibration data when processing data from the sensor unit.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of European Patent Application No14170150.8 filed on May 28, 2014, which is incorporated by reference asif fully set forth.

FIELD OF INVENTION

The present invention relates to a method, a processing unit and asensor system for determining calibration data to be used whenprocessing data from a sensor unit of the sensor system.

BACKGROUND

There are sensor system installations that require small sensor unitsbecause of restrictions in installation space or in order to enable thesensor units to be disguised. For example, in a monitoring orsurveillance installation, it may be advantageous if the camera,microphone or any other required sensor such as a magnetic sensor, issmall and thus can be disguised. One example of such installations isinstallations at ATMs (Automated Teller Machines), where at least one ofthe cameras generally is invisible or almost invisible.

Moreover, it may be advantageous to separate the sensor unit from theprocessing unit in a sensor system in order to facilitateexchangeability of the sensor unit without needing to replace theprocessing unit. For example the sensor unit may comprise limited logiccapabilities and includes as little electronics as possible, and theprocessing unit may include the more power requiring electronics and theprocessing capability of the sensor system. The exchangeability may beadvantageous for maintenance reasons or when the requirements of thesensor unit in the sensor system are changed.

When the processing unit is processing data from the sensor unit,calibration data defining specifics of the sensor unit is often requiredsince each individual sensor unit may measure the sensed physicalquantity differently compared to another sensor unit of the same sort.

A solution to this problem is described in U.S. Pat. No. 8,466,974B2(O2Micro Inc.). In this document, an identification value of the sensorunit is stored along with the calibration data for the sensor unit, forexample in a memory in the sensor unit. This identification value canthen be used for identifying which sensor unit that is connected to theprocessing unit such that the processing unit knows which calibrationdata to use. A problem with this solution is that it must be ensuredthat the identification value is unique for each sensor unit, which maybe difficult from a manufacturing point of view. Moreover, in the casethe sensor unit is recalibrated, also the identification value needs tobe updated and still be unique.

SUMMARY

In view of the above, an objective of the invention is to provide animproved way of determining calibration data to be used when processingdata from a sensor unit in a processing unit. Generally, the aboveobjective is achieved by the attached independent patent claims.

According to a first aspect, the present invention is realized by amethod performed in a processing unit for determining calibration datato be used when processing data from a sensor unit connected to theprocessing unit, the method comprising the steps of: receiving, from thesensor unit, an identifier which identifies and is calculated based oncalibration data stored in the sensor unit, checking if there is anidentifier stored in the processing unit that is identical to thereceived identifier, wherein the identifier stored in the processingunit identifies and is calculated based on calibration data stored inthe processing unit, if an identical identifier is stored in theprocessing unit, using the calibration data stored in the processingunit identified by the identical identifier when processing data fromthe sensor unit, if no identical identifier is stored in the processingunit, requesting the sensor unit to transmit calibration data, and usingthe requested calibration data when processing data from the sensorunit.

The present invention is based on the realization that an individualsensor unit can be identified by calibration data for that sensor unit.By basing the identifier on the calibration data, no specific uniquesensor identifier needs to be implemented when manufacturing the sensorunit. An advantage of the present invention is that a reduced start-uptime of a sensor system comprising the sensor unit and the processingunit may be achieved in the case the correct calibration data is alreadystored in the processing unit. This is achieved by first checking if thecalibration data for a connected sensor unit already is stored in theprocessing unit by comparing the identifiers for the stored calibrationdata in the processing unit and for the calibration data stored in thesensor unit. The identifier may require less data to be transmittedbetween the sensor unit and the processing unit for the initialverification that the correct calibration data is stored in theprocessing unit. This may be particularly advantageous in the case thesensor unit is remotely connected to the processing unit, for examplevia a bandwidth limited external link.

By performing the above method, the correctness of any calibration datastored in the processing unit may be verified in an improved way sinceonly the identifiers according to above are compared. In the case theidentifier of the stored calibration data at the processing unit is notidentical to the identifier of the calibration data stored in the sensorunit, the processing unit can request the sensor unit to transmitcalibration data, and further use the requested calibration data whenprocessing data from the sensor unit. Consequently, the processing unitmay always use the correct calibration data, i.e. the calibration datastored in the sensor unit, when processing data from the sensor unit.

An advantage of having calibration data stored in the sensor unit itselfand of having the calibration data accessed from outside the sensorunit, e.g. from the processing unit, is that installation of the sensorsystem becomes easier because the calibration data is strongly relatedto the specific sensor unit which is the subject of the calibrationdata. Moreover, since the processing unit may always verify thecorrectness of the calibration data stored in the processing unit onstart up of the sensor system, the present method facilitatesreplacement of the sensor unit connected to the processing unit. Whenreplacing a sensor unit with a new sensor unit, the new calibrationdata, i.e. the calibration data for the new sensor unit, will be loadedinto a memory of the processing unit, for example on start-up.

Moreover, by implementing the method according to this embodiment in theprocessing unit, the sensor unit can be produced and calibratedcompletely separate from the processing unit, for example in differentfactories by different companies. This may be advantageous for cost andefficiency reasons.

A further advantage is that re-calibration of the sensor unit connectedto the processing unit is facilitated. The re-calibration may mean thatnew calibration data is stored in the sensor unit, and this newcalibration data will be provided to the processing unit when the aboveis performed. Since new calibration data implies that no identicalidentifier is stored in the processing unit, new calibration data willbe requested from the sensor unit and used by the processing unit whenprocessing data from the sensor unit.

According to some embodiments, after the step of requesting the sensorunit to transmit calibration data, the method further comprises the stepof storing the thus received calibration data in the processing unit.

Consequently, the next time the processing unit is determiningcalibration data to be used when processing data from the same connectedsensor unit, for example after a restart of the sensor system, theprocessing unit will find that the identifier calculated based oncalibration data stored in the processing unit and the identifierreceived from the sensor unit are identical and no new calibration dataneeds to be transmitted from the sensor unit to the processing unit.

According to some embodiments, each of the identifier received from thesensor unit and the identifier stored in the processing unit is achecksum calculated from the calibration data it identifies. Thechecksum may be a cryptographic hash function. In other words, theactual procedure yielding each of the identifier, given the calibrationdata it identifies, may be a cryptographic hash function. Such achecksum algorithm may output identifiers with significantly differentvalues, even for small changes in the calibration data. Consequently,there is a high probability that different calibration data will yielddifferent identifiers. This is advantageous since the use of a correctcalibration data may increase the quality of any processed dataoutputted by the processing unit. Moreover, cryptographic hash functionsmay advantageously be used to detect corruption errors in thecalibration data stored in the processing unit and thus also verify thedata integrity of the calibration data stored in the processing unit.Other checksum algorithms, such as a longitudinal parity check may alsobe used.

Examples of suitable cryptographic hash functions are a MD5message-digest algorithm, a MD4 message-digest algorithm, SHA-1, SHA-2and SHA-3. Such cryptographic hash functions are typically easy tocompute based on the calibration data. Further, it may be difficult orinfeasible to modify calibration data without changing the identifier,and it may be difficult or infeasible to find two different calibrationdata which generates the same identifier. It may be noted that anysuitable cryptographic hash function may be used for calculating theidentifier(s).

According to some embodiments, a plurality of identifiers is stored inthe processing unit. In this case, the step of checking if there is anidentifier stored in the processing unit that is identical to thereceived identifier is stopped if an identical identifier is found. Anadvantage of this embodiment is that the method for determiningcalibration data may be performed more efficiently. There may be severalreasons that a plurality of identifiers is stored in the processingunit. For example, several sensor units may be connected to the sameprocessing unit. Another reason may be that several different sensorunits have been connected to the processing unit, and that theprocessing unit is configured to store a certain number of calibrationdata from previously connected sensor units.

According to some embodiments, the step of checking if there is anidentifier stored in the processing unit that is identical to thereceived identifier comprises the step of: prior to the step of checkingif a stored identifier is identical to the received identifier,calculating an identifier based on calibration data stored in theprocessing unit, temporarily storing the calculated identifier, andwherein the step of checking if there is an identifier stored in theprocessing unit that is identical to the received identifier comprisesusing the temporarily stored identifier as the identifier stored in theprocessing unit.

An advantage of calculating an identifier based on the calibration datastored in the processing unit prior to checking if the identifier isidentical to the received identifier from the sensor unit is that acorrupt calibration data stored in the processing unit may be discoveredand thus not used. Since, according to this embodiment, the identifierof the calibration data stored in the processing unit is calculatedevery time the method is performed, this means that a corrupt orotherwise changed calibration data stored on the processing unit willcause the identifier identifying this calibration data to not beidentical to the identifier received from the sensor unit. Consequently,the processing unit will request the sensor unit to transmit newcalibration data, and using the requested calibration data whenprocessing data from the sensor unit.

According to a second aspect, the present invention provides aprocessing unit configured for determining calibration data to be usedwhen processing data from a sensor unit connected to the processingunit, the processing unit comprises: a processor configured forreceiving, from the sensor unit, an identifier which identifies and iscalculated based on calibration data stored in the sensor unit, a memoryconfigured for storing calibration data and an identifier identifyingand being calculated based on the calibration data, the processor beingconfigured for checking if an identifier stored in the memory isidentical to the received identifier, for using the calibration dataidentified by the identical identifier and stored in the memory whenprocessing data from the sensor unit if the identifier stored in thememory is identical to the received identifier, and for requesting thesensor unit to transmit calibration data and using the requestedcalibration data when processing data from the sensor unit if noidentical identifier is found.

The memory in the processing unit may be any type of non-volatile memorysuch as for example ROM, PROM, EPROM, EEPROM, NVRAM, Flash memory, etc.The memory may further comprise a plurality of separate memories.

According to embodiments, the processor is further configured for, afterrequesting the sensor unit to transmit calibration data, storing thethus received calibration data in the memory.

According to embodiments, each of the identifier received from thesensor unit and the identifier stored in the memory is a checksumcalculated from the calibration data it identifies.

According to embodiments, the checksum is a cryptographic hash function.

According to embodiments, the cryptographic hash function is one of aMD5 message-digest algorithm, a MD4 message-digest algorithm, SHA-1,SHA-2 or SHA-3.

According to embodiments, a plurality of identifiers are stored in thememory, and wherein the processor is configured for stop checking ifthere is an identifier stored in the processing unit that is identicalto the received identifier if an identical identifier is found in thememory.

According to embodiments, the processor is further configured for, priorto checking if an identifier stored in the memory is identical to thereceived identifier, calculating an identifier based on calibration datastored in the memory, temporarily storing the calculated identifier inthe memory, and using the temporarily stored identifier when checking ifthere is an identifier stored in the memory that is identical to thereceived identifier.

It should be noted that when the processor is calculating an identifierbased on calibration data stored in the processing unit, the calculatedidentifier is inevitably stored in some sort of memory, both during theprocess of calculating the identifier and after the identifier has beencalculated.

According to a third aspect, the present invention provides a sensorsystem comprising a processing unit according to the second aspect and asensor unit connected to the processing unit, the sensor unitcomprising: a sensor, a memory configured to store calibration data, thecalibration data relating to properties of the sensor, wherein thememory is further configured to store an identifier which identifies andis calculated based on the calibration data, a transmitter fortransmitting the identifier stored in the memory upon request fromprocessing unit, and a transmitter for transmitting the calibration datastored in the memory upon request from the processing unit.

The transmitter for transmitting the identifier and the calibration datamay be implemented by a programmable logic device (PLD) which inaddition may act as a non-volatile memory for storing the calibrationdata and the identifier which identifies and is calculated based on thecalibration data. As an alternative to the PLD device, the transmitterfor transmitting the identifier and the calibration data may beimplemented by a FPGA device. The memory may be a separate unit, forexample a ROM, an EEPROM or Flash memory.

The second and third aspect may generally have the same features andadvantages as the first aspect.

A further scope of applicability of the present invention will becomeapparent from the detailed description given below. However, it shouldbe understood that the detailed description and specific examples, whileindicating preferred embodiments of the invention, are given by way ofillustration only, since various changes and modifications within thescope of the invention will become apparent to those skilled in the artfrom this detailed description. Hence, it is to be understood that thisinvention is not limited to the particular component parts of the devicedescribed or steps of the methods described as such device and methodmay vary. It is also to be understood that the terminology used hereinis for purpose of describing particular embodiments only, and is notintended to be limiting. It must be noted that, as used in thespecification and the appended claim, the articles “a,” “an,” “the,” and“said” are intended to mean that there are one or more of the elementsunless the context clearly dictates otherwise. Thus, for example,reference to “a sensor” or “the sensor” may include several sensors, andthe like. Furthermore, the word “comprising” does not exclude otherelements or steps.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will becomeapparent from the following detailed description of a presentlypreferred embodiment, with reference to the accompanying drawings, inwhich

FIG. 1 is a block diagram of a sensor system according to one embodimentof the invention,

FIG. 2 is a block diagram of a sensor unit and a processing unitaccording to embodiments of the invention,

FIG. 3 is a flowchart of a method for determining calibration dataaccording to one embodiment of the invention,

FIG. 4 is a flowchart of a method for determining calibration dataaccording to one embodiment of the invention.

Further, in the figures like reference characters designate like orcorresponding parts throughout the several figures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a sensor system 10. The sensor system10 may be any type of sensor system 10, for example an audio monitoringsystem or a monitoring system capturing motion video images. Accordingto one embodiment, see FIG. 1, the system includes a plurality of small(e.g. having a dimension of a few centimeters) sensor units 100, whichmay be simple cameras, microphones, accelerometers, gyroscopes,magnetometers or any other type of sensors, a processing unit 200, anddata delivery channels 300, 302, 304. The sensor unit 100 may also becalled a “thin sensor”, referring to thin client concepts in that thesmall sensor units 100 may be seen as devices which depend heavily onanother more powerful device, e.g. the processing unit 200, to fulfilltheir traditional role of sensing physical quantities of the environmentaround each sensor. This stands in contrast to traditional sensors inwhich the sensors perform all processing up to outputting a completelyprocessed data, for example a completely viewable and shareable motionvideo. The data delivery channel may be any one of, or any combinationof, a computer network 300, e.g. a WAN, a LAN, the Internet, etc., astorage device 302, e.g. a storage server, a data base, an external harddrive, etc., and a radio connection 304, e.g. a wireless LAN, mobilephone network, etc.

FIG. 2 describes by way of example a sensor unit 100 and a processingunit 200 according to embodiments of the invention.

The sensor unit 100 and the processing unit 200 are typically providedas separate units or modules. A connection 108 between the sensor unit100 and the processing unit 200 may according to some embodiments be anexternal link, for example a twisted pair cable, extending severalmeters. This is advantageous in case the sensor unit 100 needs to bepositioned far from the processing unit 200. In other embodiments, thesensor unit 100 and the processing unit 200 are separate modules orseparate units which are enclosed by a common housing or are mounted ina common support structure. The distance between the two units 100, 200may be small, e.g. measured in centimeters instead of in meters. Theconnection 108 described above is only an example, in other embodimentsthe connection 108 comprise one link for sending data from theprocessing unit 200 to the sensor unit 100 and another link for sendingdata in the other direction.

The sensor unit 100 is configured to control a sensor 102, and to packetthe raw sensor data for sending to the processing unit 200. The sensorunit is further configured to store sensor unit specific data, i.e.calibration data. A control device in the sensor unit 100 may beimplemented using a PLD 104 which in addition to acting as a controldevice may act as a non-volatile memory. The PLD 104 is thus acombination of a logic device and a memory device. The memory devicecomprises a memory which may be configured to store calibration datarelating to properties of the sensor. The calibration data is used bythe processing unit for improving sensor performance by removingstructural errors in the sensor outputs. In the case the sensor 102 isan image sensor, examples of such calibration data is information ofdead pixels in the image sensor, noise information of the image sensor,flat field correction information etc. In the case the sensor 102 is amagnetic sensor, an example of such calibration data is informationabout how the output data will change depending on the temperature ofthe sensor 102. The memory of the PLD 104 may further be configured tostore an identifier which identifies and is calculated based on thecalibration data as described above.

The logic device of the PLD 104 may be configured for, upon request fromthe processing device 200 via the link 108, transmitting the identifierstored in the memory of the PLD 104 to the processing device via thelink 108. The logic device of the PLD 104 may further be configured for,upon request from the processing device 200 via the link 108,transmitting the calibration data stored in the memory 104 to theprocessing device 200 via the link 108.

The sensor unit further comprises a serializer/deserializer 106 suchthat the sensor unit 100 can be connected to the processing unit 200 viaa serial protocol. The processing unit 200 comprises a correspondingserializer/deserializer 206.

The processing unit 200 further comprises a memory 202. The memory 202may be configured for storing calibration data regarding a currently orpreviously connected sensor unit 100. The processing unit 200 furthercomprises a processor 204. The processor 204 may be configured to removeany stored calibration data from the memory 202 when storing newcalibration data in the memory 202. This may be advantageous since lessmemory 202 may be required in the processing unit 200. According toother embodiments, the processor is configured store a certain number ofunique calibration data in the memory 202.

The memory 202 may further be configured for storing an identifieridentifying and being calculated based on the calibration data stored inthe memory 202. According to some embodiments, this identifier iscalculated by the processor and stored in the memory 202 when thecalibration data is stored in the memory 202. The advantage of this maybe that the identifier only needs to be calculated once, which savescomputational resources. According to other embodiments, the processorstores the identifier received by the connected sensor unit 100, e.g. instep S404 in FIG. 3. Consequently, the identifier never has to becalculated by the processor 204 which saves computational resources. Afurther advantage of this embodiment is that the received identifierS404 can be used for validating a subsequently received calibrationdata, as will be further described below. According to yet anotherembodiment, the processor 204 calculates the identifier every time ithas to validate that the correct calibration data is stored in thememory 202 and should be used when processing data from the sensor unit100. According to this embodiment, the identifier identifying thecalibration data stored in the memory 202 is only temporarily stored inthe memory 202. This may be advantageous in that it can be discovered ifthe calibration data stored in the memory 202 is corrupt or otherwisechanged since it was last used, as will be described in detail below.

The processor 204 will now be further described in conjunction with FIG.3 describing a method for determining calibration data to be used whenprocessing data from the sensor unit 100 connected to the processingunit 200. The method may be performed at any time by a processing unit200, but usually it is performed when the sensor system 10 is started orwhen a new sensor unit 100 is connected to the processing unit 200. Whenfor example the sensor system 10 is started, the processor 204 may beconfigured for requesting S402 the connected sensor unit 100 to transmitan identifier which identifies and is calculated based on calibrationdata stored in the sensor unit 100. When the identifier is receivedS404, the processor 204 may be configured for checking S406 if anidentifier stored in the memory 202 is identical to the receivedidentifier. In the case where a plurality of identifiers is stored inthe memory 202, i.e. in the case where a plurality of calibration datais stored in the memory 202, the processor 204 may be configured to stopchecking if there is an identifier stored in the memory 202 that isidentical to the received identifier if an identical identifier is foundin the memory 202. The calibration data indentified by the identicalidentifier is then used when processing S410 data from the sensor unit.

The processor may further be configured for using the calibration dataidentified by the identical identifier and stored in the memory 202 whenprocessing data from the sensor unit 100 if the identifier stored in thememory 202 is identical to the received identifier. This means that thecurrently connected sensor unit 100 has been connected to the processingunit 100 before and consequently the calibration data relating to thecurrently connected sensor unit is already stored in the memory 202 ofthe processing unit 200. The calibration data to be used when processingdata from the sensor unit 100 has thus been determined by onlytransmitting a small amount of data between the processing unit 200 andthe sensor unit 100. Consequently, an efficient way of determiningcalibration data to be used when processing data from a sensor unit isachieved.

The, by the processing unit 204, processed data may be transmitted, viaa network interface 208, to a data delivery channel, for example acomputer network 300.

Each of the identifier received from the sensor unit 100 and theidentifier stored in the processing unit 200 may be a checksumcalculated from the calibration data it identifies.

Consequently, in order to save time at start-up of the sensor system 10or when a sensor unit 100 is unplugged and then plugged in again thesensor unit 100 comprises an identification code identifying the presentcalibration data of the sensor unit, e.g. the checksum of thecalibration data. When the sensor unit 100 is plugged in the checksum istransferred S404 to the processing unit 200 where it is compared S406with the checksum of calibration data presently stored in the processingunit 200. If the checksums are identical, then the calibration dataalready stored in the processing unit 200 may be used.

The processor 204 may further be configured for requesting S408 thesensor unit 100 to transmit calibration data and using the requestedcalibration data when processing data from the sensor unit 100 if noidentical identifier, e.g. checksum, is found. Consequently, the correctcalibration data is used when processing data from the sensor unit 100.In the case the received identifier S404 is stored in the memory 202 bythe processor 204, the calibration data transmitted by the sensor unit100 can be verified. If the transmitted calibration data is received ina corrupt state, this will be noticed by the processor 204, which thencan take appropriate actions. For example, the processor 204 can beconfigured for requesting S408 the sensor unit 100 to transmit thecalibration data again. Such verification of the received calibrationdata may be performed by comparing the received identifier with anidentifier calculated from the received calibration data by theprocessing unit 200.

The correctness of the used calibration data may be further ensured bycalculating the checksum with a cryptographic hash function such as aMD5 message-digest algorithm, a MD4 message-digest algorithm, SHA-1,SHA-2 or SHA-3. Such functions have the advantage that the identifierwill appear random, e.g. since small differences in the calibration datait identifies may cause the identifier to have a significantly differentvalue. The integrity of the calibration data may thus be improved.Further, since small differences in the calibration data causessignificantly different identifiers, such a small differences is likelyto be discovered and increase the likelihood that the correctcalibration data is used by the processing unit 200 when processing dataS410 from the sensor unit 100.

FIG. 4 describes a further embodiment of the method of determiningcalibration data to be used when processing data from the sensor unit100 connected to the processing unit 200. The two first steps S402, andS404 are identical with the steps described in conjunction with FIG. 3.However, the method described in FIG. 4 comprises an extra step S405prior to the step of checking S406 if there is an identifier stored inthe processing unit 200, e.g. in the memory 202, that is identical tothe received identifier. The additional step comprises calculating S405an identifier based on calibration data stored in the memory 202 andtemporarily storing the calculated identifier. According to thisembodiment, the step of checking S406 if there is an identifier storedin the processing unit 200, e.g. in the memory 202, that is identical tothe received identifier comprises using the temporarily storedidentifier as the identifier stored in the processing unit 200.

According to the embodiment described in FIG. 4, the method comprisesanother additional step S409 compared to the embodiment described inFIG. 3. In the case the step of checking S406 if there is an identifierstored in the processing unit that is identical to the receivedidentifier results in a negative answer, and the processor 204consequently requests the sensor unit 100 to transmit calibration data,the present embodiment comprises the step of storing S409 the thusreceived calibration data in the processing unit 200, e.g. in the memory202.

Moreover, the skilled person would realize that the method in itsvarious embodiments described in conjunction with FIGS. 1-4 may beimplemented in a computer program product comprising a computer-readablemedium with instructions for performing said method.

What is claimed is:
 1. A method performed in a processing unit fordetermining calibration data to be used when processing data from asensor unit connected to the processing unit, the method comprising:receiving, from the sensor unit, an identifier which identifiescalibration data stored in the sensor unit, the identifier beingcalculated based on the calibration data stored in the sensor unit;checking if there is an identifier stored in the processing unit that isidentical to the received identifier from the sensor unit to determinewhether to use the calibration data identified by the stored identifierwhen processing data from the sensor unit without requesting calibrationdata from the sensor unit, wherein the identifier stored in theprocessing unit identifies calibration data stored in the processingunit, the identifier stored in the processing unit being calculatedbased on the calibration data stored in the processing unit; using thecalibration data stored in the processing unit identified by theidentical identifier when processing data from the sensor unit inresponse to an identical identifier being stored in the processing unit;and requesting the sensor unit to transmit calibration data, and usingthe requested calibration data when processing data from the sensor unitin response to an identical identifier not being stored in theprocessing unit.
 2. The method according to claim 1, wherein afterrequesting the sensor unit to transmit calibration data, the methodfurther comprises storing the thus received calibration data in theprocessing unit.
 3. The method according to claim 1, wherein each of theidentifier received from the sensor unit and the identifier stored inthe processing unit is a checksum calculated from the calibration datait identifies.
 4. The method according to claim 3, wherein the checksumis a cryptographic hash function.
 5. The method according to claim 4,wherein the cryptographic hash function is one of a MD5 message-digestalgorithm, a MD4 message-digest algorithm, SHA-1, SHA-2 or SHA-3.
 6. Themethod according to claim 1, wherein a plurality of identifiers isstored in the processing unit, and wherein the checking if there is anidentifier stored in the processing unit that is identical to thereceived identifier is stopped if an identical identifier is found. 7.The method according to claim 1, wherein the checking if there is anidentifier stored in the processing unit that is identical to thereceived identifier comprises: prior to the step of checking if a storedidentifier is identical to the received identifier; calculating anidentifier based on calibration data stored in the processing unit; andtemporarily storing the calculated identifier, wherein the checking ifthere is an identifier stored in the processing unit that is identicalto the received identifier comprises using the temporarily storedidentifier as the identifier stored in the processing unit.
 8. Aprocessing unit configured for determining calibration data to be usedwhen processing data from a sensor unit connected to the processingunit, the processing unit comprises: a processor configured forreceiving, from the sensor unit, an identifier which identifiescalibration data stored in the sensor unit, the identifier beingcalculated based on the calibration data stored in the sensor unit; amemory configured for storing calibration data and an identifieridentifying calibration data stored in the processing unit, theidentifier stored in the memory being calculated based on thecalibration data stored in the processing unit; the processor beingconfigured for checking if an identifier stored in the memory isidentical to the received identifier to determine whether to use thecalibration data identified by the stored identifier when processingdata from the sensor unit without requesting calibration data from thesensor unit, for using the calibration data identified by the identicalidentifier and stored in the memory when processing data from the sensorunit in response to the identifier stored in the memory being identicalto the received identifier, and for requesting the sensor unit totransmit calibration data and using the requested calibration data whenprocessing data from the sensor unit in response to no identicalidentifier being found.
 9. The processing unit according to claim 8,wherein the processor is further configured for, after requesting thesensor unit to transmit calibration data, storing the thus receivedcalibration data in the memory.
 10. The processing unit according toclaim 8, wherein each of the identifier received from the sensor unitand the identifier stored in the memory is a checksum calculated fromthe calibration data it identifies.
 11. The processing unit according toclaim 10, wherein the checksum is a cryptographic hash function.
 12. Theprocessing unit according to claim 11, wherein the cryptographic hashfunction is one of a MD5 message-digest algorithm, a MD4 message-digestalgorithm, SHA-1, SHA-2 or SHA-3.
 13. The processing unit according toclaim 8, wherein a plurality of identifiers is stored in the memory, andwherein the processor is configured for stop checking if there is anidentifier stored in the processing unit that is identical to thereceived identifier if an identical identifier is found in the memory.14. The processing unit according to claim 8, wherein the processor isfurther configured for, prior to checking if an identifier stored in thememory is identical to the received identifier, calculating anidentifier based on calibration data stored in the memory, temporarilystoring the calculated identifier in the memory, and using thetemporarily stored identifier when checking if there is an identifierstored in the memory that is identical to the received identifier.
 15. Asensor system comprising: a processing unit; and a sensor unit connectedto the processing unit; wherein the processing unit comprises: aprocessor configured for receiving, from the sensor unit, an identifierwhich identifies calibration data stored in the sensor unit, theidentifier being calculated based on the calibration data stored in thesensor unit; a memory configured for storing calibration data and anidentifier identifying calibration data stored in the processing unit,and the identifier stored in the memory being calculated based on thecalibration data stored in the processing unit; the processor beingconfigured for checking if an identifier stored in the memory isidentical to the received identifier to determine whether to use thecalibration data identified by the stored identifier when processingdata from the sensor unit without requesting calibration data from thesensor unit, for using the calibration data identified by the identicalidentifier and stored in the memory when processing data from the sensorunit in response to the identifier stored in the memory being identicalto the received identifier, and for requesting the sensor unit totransmit calibration data and using the requested calibration data whenprocessing data from the sensor unit in response to no identicalidentifier being found; wherein the sensor unit comprises: a sensor; amemory configured to store calibration data, the calibration datarelating to properties of the sensor, wherein the memory is furtherconfigured to store an identifier which identifies calibration datastored in the sensor unit and is calculated based on the calibrationdata; a transmitter for transmitting the identifier stored in the memoryupon request from processing unit; and a transmitter for transmittingthe calibration data stored in the memory upon request from theprocessing unit.